Dominance of opportunistic generalist species after aquatic microbial community coalescence

Abstract Coalescence describes the mixing of two entire communities, a process frequently encountered by naturally occurring microbial communities. Such coalescence events can significantly impact microbial community dynamics, and previous studies have highlighted the importance of diversity, productivity of the mixing communities, and coherence among their members in predicting post‐coalescence assembly dynamics. However, little attention has been paid to how the distribution of life history traits within the mixing communities influences the coalescence outcome. Life history traits that determine how species respond to environmental change may be especially relevant when coalescence events coincide with environmental shifts, as is typically observed when two aquatic habitats mix. In our study, we experimentally mixed communities from two aquatic sites with contrasting levels of eutrophication and disturbance histories. We incubated them for 5 days in two different media, each varying in the quality of supplied organic matter. We used metabarcoding sequencing combined with functional measurements to characterize the communities' structural characteristics and assess their performance after mixing and transplanting them into the incubation media. Our results showed that the eutrophic community, characterized by low diversity but dominated by fast‐growing generalists, strongly influenced the post‐coalescence community in terms of productivity and composition. However, diversity metrics and elevated respiration rates in the coalesced communities suggest that rare members from the specialist and highly diverse oligotrophic community were present, potentially exerting an antagonistic effect on the dominant members of the eutrophic community. Overall, our data suggest a strong impact of life histories on the community dynamics in aquatic post‐coalescence communities.